Is it possible to delete linked Mesh Data?
Not directly. I have written a script that imports and exports meshes, when the unlinked data hits a critical amount of objects (around 50k) Blender slows down like crazy. I ended up restarting it for each cycle. All I found out - and I talked to a lot of people, even at Bcon - that's not supported. I would love to be proven wrong, though. 
However you can delete them by re-loading your file. Make sure there is a 0 in front of your datablock. Every datablock that has no users (materials, images etc. as well) will be deleted on closing the file. So if you delete every object that has the according mesh attached, the datablock will be deleted next time you open the file. 
I don't know what you want to achieve by using 'make single user', but selecting a couple of objects and then the one with worldspawn_16740 last, pressing CRTL L -&gt; link object data will have the same effect as deleting the objects in regard to their meshes.
EDIT: Looks like more ppl had the same problem and as so often the devs listened. In Blender 2.8+ you can do this: 

I know that doesn't help with batch setting the users to 0, but I'm just happy I found this.